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The scheme of the pharyngeal arch - adapted from Gray's anatomy.
In the development of vertebrates, the pharyngeal arches (which develop into the branchial arches or gill arches in fish) are primordia for a multitude of structures. In the human embryo (where the vasculature of the pharyngeal arches is also known as the aortic arches), they develop during the fourth week as a series of mesodermal outpouchings on both sides of the developing pharynx. In fish, the branchial arches support the gills.
In vertebrates, the pharyngeal arches are derived from all three germ layers. Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage. However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development. The first, most anterior pharyngeal arch gives rise to the oral jaw. The second arch becomes the hyoid and jaw support. In fish, the other posterior arches contribute to the branchial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus. The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that Hox genes and other developmental genes such as DLX are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches. Some fish species have a second set of jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation.
During human and all vertebrate development, a series of pharyngeal arch pairs form in the developing embryo. These project forward from the back of the embryo toward the front of the face and neck. Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. The arches are numbed from 1 to 6, with 1 being the arch closest to the head of the embryo, and arch 5 existing only transiently.:318–323
These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.
Each pharyngeal arch has a cartilaginous stick, a muscle component that differentiates from the cartilaginous tissue, an artery, and a cranial nerve. Each of these is surrounded by mesenchyme. Arches do not develop simultaneously but instead possess a "staggered" development.
Pharyngeal pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. In fish the pouches line up with the clefts, and these thin segments become gills. In mammals the endoderm and ectoderm not only remain intact but also continue to be separated by a mesoderm layer.
The development of the pharyngeal arches provides a useful landmark with which to establish the precise stage of embryonic development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger-Hamilton stages 14 to 28 in the chicken. Although there are six pharyngeal arches, in humans the fifth arch exists only transiently during embryogenesis.
Since no human structures result from the fifth arch, the arches in humans are I, II, III, IV, and VI. More is known about the fate of the first arch than the remaining four. The first three contribute to structures above the larynx, whereas the last two contribute to the larynx and trachea.
The recurrent laryngeal nerves are produced from the nerve of arch 6, and the laryngeal cartilages from arches 4 and 6. The superior laryngeal branch of the vagus nerve arises from arch 4. Its arteries, which project between the nerves of the fourth and sixth arches, become the left-side arch of the aorta and the right subclavian artery. On the right side, the artery of Arch 6 is obliterated while, on the left side, the artery persists as the ductus arteriosus; circulatory changes immediately following birth cause the vessel to close down, leaving a remnant, the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths.:318–323
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